Conventionally, in an asynchronous successive approximation register (SAR) analog-to-digital converter (ADC), an analog-to-digital (AD) conversion time per cycle is determined by applying a predetermined delay time to an output signal of a comparator. Such delay time is determined in advance by, for example, a fixed delay circuit. In the case where the AD conversion time necessary per cycle becomes long, the SARADC using the fixed delay circuit may fall below a predetermined number of cycles with respect to the overall AD conversion time (i.e., resolution is degraded).
In contrast, a SARADC capable of appropriately adjusting a delay time is known. Such SARADC uses a delay circuit which is capable of adjusting the delay time appropriately, and adjusts the delay time by a ring oscillator and a counter. The time delay is adjusted by adjusting an oscillating frequency of the ring oscillator. The oscillating frequency of the ring oscillator is adjusted by a feedback loop so that the value of a counter becomes a predetermined value. According to this SARADC, it is possible to adjust the delay time while AD conversion is in operation (i.e., background adjustment). However, since high-speed operation would be required for the ring oscillator and the counter, there is a problem that large power consumption is required.
As another SARADC, a configuration which adjusts a delay time by counting an AD conversion cycle of a predetermined time is known. Such SARADC detects an output edge of a comparator and increases the value of a counter for the number of detected edges. The value of the counter corresponds to the number of cycles of an AD conversion that occurred during the overall AD conversion time. In other words, this SARADC determines the delay time as being short if the value of the counter is larger than a predetermined value, and as being long if the value of the counter is smaller than the predetermined value. However, since this SARADC performs control by an accumulated value of the value of the counter, there is a problem that a time for adjustment would be required separately (i.e., background adjustment cannot be performed).